Microwave landing systems (MLS) facilitate aircraft approach and landing operations. Such systems include so-called precision distance measuring equipment (DME/P), which provides aircraft distance information by measuring total round-trip time between pulse-pair interrogations from an airborne transmitter and replies from a ground transponder. This type of system is capable of providing high accuracy ranging information in the severe multipath environment encountered during approach and landing operations.
In operation, a DME/P receiver located at the ground transponder receives a pulse-pair interrogation from an airborne transmitter. To enable the system to provide ranging information for a large number of aircraft, the system utilizes a plurality of channels allocated along a frequency spectrum. Each pulse-pair interrogation is normally assigned to a specific frequency channel, with each channel spaced approximately 1 MHz apart.
A proper response to the pulse-pair interrogation from a specific airborne transmitter requires a determination of the frequency of the pulse-pair. In the prior art, such frequency determinations have been made, for example, by a so-called "Ferris discriminator" circuit. This frequency discriminator circuit employs a narrowband filter, whose output detects the presence of an interrogation pulse, in combination with a wideband filter, whose output is gated only when there is also in coincidence a valid output from the narrowband filter. Thus in the Ferris discriminator, the output of the narrowband filter must exceed a predetermined output level of the wideband filter for a so-called "on-channel" condition to be declared. An "on-channel" condition indicates that the frequency of the received pulses is within the bandwidth of the narrowband filter.
A conventional DME/P receiver operates on logarithmically-detected video signals. The Ferris discriminator, however, operates on linearly-detected RF signals. It is therefore impractical to use a Ferris discriminator circuit in a DME/P receiver, because the on-channel determination provided by the Ferris circuit depends on the difference between two linear signals, and there is no equivalent for such signals in the logarithmic domain. Indeed, such use requires additional IF signal processing circuitry, which increases discriminator cost and complexity, and decreases overall system reliability. Moreover, at low input signal levels, receiver noise adversely affects operation of the Ferris discriminator circuit, thus further degrading receiver operation.
There is therefore a need to provide an improved frequency discriminator circuit which may be efficiently and economically used in the DME/P receiver of a microwave landing system.